Mammalian chromosomal fragments from tissue culture cell lines such as rodent minimal deviation hepatoma will be hybridized to normal and phenylketonuric fibroblasts which do not have phenylalanine hydroxylase activity. Hybridization will be followed by selection in tyrosineless medium. This maneuver will select those colonies that have acquired phenylalanine hydroxylase activity. The phenotype of the hybrid cell line will be examined by isoelectric focussing and immunological investigations to determine if the human genome for the phenylalanine hydroxylase locus has been activated. BIBLIOGRAPHIC REFERENCES: Tourian, A. Control of Phenylalanine hydroxylase synthesis in tissue culture by serum and insulin (in press). J. Cell. Physiol. 1976. Tourian, A. The unique identity of rat hepatoma phenylalanine hydroxylase. Bioch. Bioph. Res. Comm. 68:51, 1976.